TYPES OF WEAR

In most basic wear studies where the problems of wear have been a primary concern, the so-called dry friction has been investigated to avoid the influences of fluid lubricants.

Dry friction’ is defined as friction under not intentionally lubricated conditions but it is well known that it is friction under lubrication by atmospheric gases, especially by oxygen [20].

A fundamental scheme to classify wear was first outlined by Burwell and Strang [21].

Later Burwell [22] modified the classification to include five distinct types of wear, namely (1) Abrasive (2) Adhesive (3) Erosive (4) Surface fatigue (5) Corrosive.

2.8.1 Abrasive wear

Abrasive wear can be defined as wear that occurs when a hard surface slides against and cuts groove from a softer surface. It can be account for most failures in practice. Hard particles or asperities that cut or groove one of the rubbing surfaces produce abrasive wear.

This hard material may be originated from one of the two rubbing surfaces. In sliding mechanisms, abrasion can arise from the existing asperities on one surface (if it is harder than the other), from the generation of wear fragments which are repeatedly deformed and hence get work hardened for oxidized until they became harder than either or both of the sliding surfaces, or from the adventitious entry of hard particles, such as dirt from outside the system.

Two body abrasive wear as shown in fig. 2.3 occurs when one surface (usually harder than the second) cuts material away from the second, although this mechanism very often changes to three body abrasion as the wear debris then acts as an abrasive between the two surfaces. Abrasives can act as in grinding where the abrasive is fixed relative to one surface or as in lapping where the abrasive tumbles producing a series of indentations as opposed to a scratch. According to the recent tribological survey, abrasive wear is responsible for the largest amount of material loss in industrial practice [23].

2.8.2 Adhesive wear

Adhesive wear can be defined as wear due to localized bonding between contacting solid surfaces leading to material transfer between the two surfaces or the loss from either surface. For adhesive wear as shown in fig. 2.4 to occur it is necessary for the surfaces to be in intimate contact with each other. Surfaces, which are held apart by lubricating films, oxide films etc. reduce the tendency for adhesion to occur.

Fig. 2.3Schematic representations of the abrasion wear mechanism.

Fig. 2.4Schematic representations of the adhesive wear mechanism.

2.8.3 Erosive wear

Erosive wear can be defined as the process of metal removal due to impingement of solid particles on a surface. Erosion is caused by a gas or a liquid, which may or may not carry, entrained solid particles, impinging on a surface. When the angle of impingement is small, the wear produced is closely analogous to abrasion. When the angle of impingement is

normal to the surface, material is displaced by plastic flow or is dislodged by brittle failure.

The schematic representation of the erosive wear mechanism is shown in fig.2.5.

2.8.4 Surface fatigue wear

Wear of a solid surface caused by fracture arising from material fatigue. The term

‘fatigue’ is broadly applied to the failure phenomenon where a solid is subjected to cyclic loading involving tension and compression above a certain critical stress. Repeated loading causes the generation of micro cracks, usually below the surface, at the site of a pre-existing point of weakness. On subsequent loading and unloading, the micro crack propagates. Once the crack reaches the critical size, it changes its direction to emerge at the surface, and thus flat sheet like particles is detached during wearing. The number of stress cycles required to cause such failure decreases as the corresponding magnitude of stress increases. Vibration is a common cause of fatigue wear. The schematic representation of the surface fatigue wear mechanism is shown in fig. 2.6.

Fig. 2.5 Schematic representations of the erosive wear mechanism.

Fig. 2.6 Schematic representations of the surface fatigue wear mechanism.

2.8.5 Corrosive wear

Most metals are thermodynamically unstable in air and react with oxygen to form an oxide, which usually develop layer or scales on the surface of metal or alloys when their interfacial bonds are poor. Corrosion wear is the gradual eating away or deterioration of unprotected metal surfaces by the effects of the atmosphere, acids, gases, alkalis, etc. This type of wear creates pits and perforations and may eventually dissolve metal parts.

2.9 SYMPTOMS OF WEAR

Literature available on the rate controlling wear mechanism demonstrated that it may change abruptly from one another at certain sliding velocities and contact loads, resulting in abrupt increases in wear rates. The conflicting results in the wear literature arise partly because of the differences in testing conditions, but they also make clear that a deeper understanding of the wear mechanism is required if an improvement in the wear resistances of the coating is to be achieved. This in turn requires a systematic study of the wear under different stresses, velocities and temperatures. It is generally recognized that wear is a characteristic of a system and influenced by many parameters. Laboratory scale investigation if designed properly allows careful control of the tribo system where by the effects of different variables on wear behaviour of the coating can be isolated and determined. The data generated through such investigation under controlled conditions may help in correct interpretation of the results.

A summary of the appearance and symptoms of different wear mechanism is indicated in Table 2.2 and the same is a systematic approach to diagnose the wear mechanisms.

Types of wear Symptoms Appearance of the worn-out surface

Abrasive Presence of clean furrows cut out by abrasive particles

Grooves

Adhesive Metal transfer is the prime symptoms Seizure, catering rough and torn- out surfaces.

Erosion Presence of abrasives in the fast moving fluid and short abrasion furrows

Waves and troughs.

Corrosion Presence of metal corrosion products. Rough pits or depressions.

Fatigue Presence of surface or subsurface cracks accompanied by pits and spalls

Sharp and angular edges around pits.

Impacts Surface fatigue, small sub micron particles or formation of spalls

Fragmentation, peeling and pitting.

Delamination Presence of subsurface cracks parallel to the surface with semi-dislodged or loose flakes

Loose, long and thin sheet like particles

Fretting Production of voluminous amount of loose debris

Roughening, seizure and development of oxide ridges

Electric attack Presence of micro craters or a track with evidence of smooth molten metal

Smooth holes

Table 2.2 Symptoms and appearance of different types of wear [24].

A typical model, exemplifying the rate of erosion depending on size and velocity of particle on impacting the substrate is shown in fig.2.7. The increase in impact velocity or particle diameter clearly accelerates erosion damage. From the fact that an increase in particle velocity or size leads to larger or deeper indentations as schematically shown in Fig. 2.7, deviations in k2 and k3 values from the theoretical ones (k2 =2, k3 = 0) indicate the true effects of impact velocity and particle diameter which are connected with the relative aggressiveness of indentation. The larger or deeper is the indentation the greater amount of material is removed from the rim of the indentation.

Fig. 2.7 Model of the effects of impact parameters on exponents k2 and k3.